One method of measuring the state of charge of a battery is to test the specific gravity of the electrolyte in each cell of the battery. When the battery is fully charged, the electrolyte will have a specific gravity of about 1.25 to 1.29, meaning the electrolyte will weigh 1.25 to 1.29 times as much as the same volume of pure water. A battery that is less than fully charged will have a correspondingly lower specific gravity.
This test is performed with a hydrometer which has a small float in a glass barrel and a rubber bulb at the top of the barrel. The bulb must be squeezed and released to draw some of the electrolyte into the barrel and the level of the float in the barrel measured to determine the specific gravity. This test, therefore, is capable of determining the state of charge of a battery whose cells are accessible and which is not in use. Many of today's batteries, however, are sealed tight and their cells are not accessible. Furthermore, there are many instances where this test is inconvenient to perform. For example, where a large bank of batteries is used, it is not practical to measure the specific gravity of each cell in every battery. Also when an automobile is in use, for example, in a traffic jam, it is not practical to determine the battery state of charge with a hydrometer.
An attempt to overcome the inconvenience associated with testing the specific gravity with a hydrometer resulted in the installation of a hydrometer in a single cell of various automotive batteries to measure the specific gravity of that cell. If that cell has sufficient charge, then a green dot appears at the top of the battery. Automotive batteries, however, have six cells and the total output of such a battery is the average of all six cells. This device is, therefore, capable of monitoring the state of charge of only a single cell in a battery which may not be indicative of the overall charge stored in a battery.
Voltmeters and ammeters have been used to measure the output from a battery, however, each of these meters only measures the instantaneous voltage or current, respectively. They are not able to integrate the result and, therefore, can not indicate the charge stored in a battery, unless these meters are monitored every instant.
A current counter which continuously registers the net current from an automotive battery is disclosed in the ELV Journal No. 45, dated May/June 1986. This device measures the voltage drop across a precision shunt resistor, which is in series with the monitored battery. The voltage is amplified and drives a voltage to frequency converter, the output of which is fed into an up/down counter. The counter counts up or down while the battery is discharging or being charged, respectively. The output of the counter drives an integrated display.
The display is initialized when the current counter is first connected to a fully charged battery and from there on indicates the net current flow from the battery. For continuous monitoring, however, the circuitry leading to the up/down counter must be continuously energized by a power source, such as the battery, since there is no way for the ELV current counter to remember the net current from a battery if it loses power.